I lose track of many books that might have caught my attention for a brief, shiny moment. The Goodreads app makes it simple for me to pop titles into a "might want to check this out someday" shelf when browsing newsfeeds and "what are you reading now?" threads. It keeps the slush-pile so I don't have to. My "reviews" are probably not very useful to anyone else, but GoodReads will also tweet my reviews/ratings/quotes/status updates to my 3 Twitter followers automatically--thus saving me the effort of having to do so manually. It mostly works to amuse me.

I don't know if there's any one thing all users are "supposed" to do on Goodreads or use it for.

I use it to:

1) Catalog my extensive collection. It got to the point that I couldn't remember always what I did or didn't have, especially for old, OP series. Helpful for insurance purposes too.
2) Rate & review books I've read.
3) Interact with like minded readers in several groups
4) Try to get some free giveaways. Gotten a few, but so many people want them, I wouldn't count on it.
5) Keep track of my want list and current reading.
6) I even became a Goodreads librarian, mostly to help correct listings of items from my collection (missing covers, mostly)

BTW as someone said, there are a number or 'ways' of using goodreads. I mostly participate in the Science, Science Fiction, and Short Stories discussion groups. I've listed some books and ratings etc, but nothing like all I've read. I use it mostly to get recommendations, reviews and discussion on short story and science topics.

I use it mostly to record the books I've read and my ratings. Don't do many review. I suck at reviews

LOL! Same here. I like reading people's reviews but I myself never post any. I use Goodreads to rate the books I've read and to add books that I want to read. I look at the recommendations, follow my favorite authors, I like looking through the Listopia section, stuff like that